An automatic transmission mounted on a vehicle is configured to combine a torque converter and a transmission mechanism and selectively fasten a plurality of friction elements such as a clutch and a brake according to an operation state to thereby change an actuation state of the transmission mechanism, that is, a power transmission route and automatically change gear to a predetermined gear shift stage.
For example, Patent Literature 1 discloses an automatic transmission in which a low reverse brake and a one-way clutch are arranged in parallel and, in forward first gear speed, only a low clutch, which is fastened to input output rotation on an engine side to a transmission mechanism, is fastened, whereby the one-way clutch is locked to form a power transmission route of the forward first gear speed. Note that Patent Literature 1 mentions that the low reverse brake is fastened in reverse speed and is fastened even in the forward first gear speed if an engine brake is necessary when stepping of an accelerator pedal is released and the engine side is reversely driven from a wheel side.
However, since the one-way clutch is provided, the cost, the weight, and the size of the automatic transmission increase. Moreover, in gear shift stages other than a starting gear shift stage, since the one-way clutch idles without being locked, drag resistance occurs and deterioration in fuel efficiency is caused. Therefore, it is proposed to dispense with the one-way clutch and, in the forward first gear speed, always fasten the low clutch and the low reverse brake. However, Patent Literature 1 does not disclose a specific fastening operation in that case. Instead, Patent Literature 1 describes the influence of centrifugal hydraulic pressure in fastening the low clutch. That is, since the low clutch is fastened to input output rotation on the engine side to the transmission mechanism, the low clutch is always rotating with the output rotation on the engine side during the operation of the engine. According to this rotation, hydraulic oil in a hydraulic pressure chamber of the low clutch receives a centrifugal force and the pressure of the hydraulic oil rises. This pressure rise affects fastening control for the low clutch. Therefore, Patent Literature mentions that a balance chamber is partitioned on an opposite side of a hydraulic chamber of a piston and the hydraulic oil is supplied to the balance chamber as well, whereby the influence of the centrifugal hydraulic pressure is offset.
Incidentally, since the starting gear shift stage has a large reduction ratio, an unpleasant shock tends to occur if timing of the fastening of the low clutch deviates. Therefore, it is more important to eliminate the influence of the centrifugal hydraulic pressure. However, under the current situation, the influence of the centrifugal hydraulic pressure cannot be sufficiently eliminated simply by providing the balance chamber.